Combined cycle power plants generally include a gas turbine driven by combusting fuel, such as natural gas, and a heat recovery steam generator that produces steam from the exhaust of the gas turbine to power a steam turbine. Therefore, some of the heat generated during the operation of the gas turbine may be captured by the heat recovery steam generator to produce steam that can be used for further generation of electrical power. More particularly, the heat from the operation of the gas turbine may be captured from the exhaust of the gas turbine. The gas turbine may be operated in one of several different firing modes, or dry low NOx (DLN) mode, where each firing mode may correspond to a varying number of combustors, or cans, of the gas turbine that are operated. The quantity of fuel and/or the fuel air mix that is combusted in the gas turbine may vary for each of the firing modes. If the operation of the gas turbine is changed from one firing mode to another firing mode, the temperature of the exhaust may also change.
To enable efficient operation of the steam turbine, and therefore the combined cycle power plant, the steam temperature may be controlled within a predetermined range. Manufacturers of power plants may provide a specification related to the level of control of the steam that is achievable by the combined cycle power plant. In some cases, the manufacturers of combined cycle power plants may be required to demonstrate the control of steam temperature with the predetermined range. The steam temperature may be controlled by a variety of mechanisms, including attemperation by spraying water into a counter flowing heat exchanger of the heat recovery steam generator. The amount of water that is provided may be modulated to control the temperature of the steam.
In certain cases, it may be difficult to control the temperature within a predetermined band, such as a band of 10° Fahrenheit (F). For example, it may be particularly difficult to control the temperature of the steam generated by the heat recovery steam generator when the gas turbine changes firing modes. The discontinuity in the output temperature of the exhaust gases from the gas turbine as a result of the change in firing mode may be difficult to compensate for using water spray attemperation within the heat recovery steam generator. Therefore, changes in the firing mode of the gas turbine may result in temperature excursions outside of allowable limits of temperature of the steam generated by the heat recovery steam generator.